If you email otter, I know he has many older spyders aswell as newer ones, he could probably tell you if the older frames worked on newer guns.

Threadings wouldn't matter between two frames, because the screws don't actually screw into the frame, they only hold it on, so my guess is that either the position of the sear is different or the frame is differently shaped or screw holes aren't in the same positions. Thats just on the assumption that the older frames aren't compatible. I'd still email otter to verify that.